Lithium-ion secondary batteries are lightweight and have excellent input and output characteristics as compared to conventional secondary batteries such as nickel cadmium batteries, nickel hydride batteries, and lead-acid batteries, and are thus practically used as drive power supplies for portable electronic devices such as mobile phones and notebook computers.
Negative electrodes of lithium-ion secondary batteries are generally made of carbon materials into which and from which lithium ions are inserted and released. Graphite materials are the mainstream of the carbon materials in view of providing flat discharge potential and high capacity density.
A report says that it is easy to stably form an intercalation compound from a graphite material and lithium, as the crystal structure of graphite grows. A large amount of lithium is inserted between graphene sheets, thereby obtaining high discharge capacity (e.g., Non-Patent Document 1).
On the other hand, in a lithium-ion secondary battery including a negative electrode made of a graphite material is subject to side reaction such as decomposition of an electrolytic solution, which is not related to the reaction of the battery, at the graphite surface in initial charging as the crystallinity of the graphite increases. Thus, with the increasing crystallinity of the graphite, irreversible capacity greatly increases, which is obtained by subtracting initial discharge capacity from initial charge capacity and cannot be extracted as electricity in subsequent charging and discharging. This causes loss of discharge capacity ranging from tens to hundreds of mAh/g in initial discharging (see, e.g., Non-Patent Document 2).
In order to address the problem, processes of forming the following substantially layered structures have been suggested and repeatedly modified. The surface of a highly crystalline graphite material is coated with low-crystalline carbon using thermal decomposition gas made of an organic compound such as propane and benzene (see, e.g., Patent Document 1). A graphite precursor is mechanochemically treated and then graphitized to decrease the crystallinity of the surface relative to the crystallinity of the nucleus (see, e.g., Patent Document 2).